Thanks for your replies and information - it’s helping me get my head around it all!
I think I had over 10k NSR, but I’m not sure it was in one address at the time of the issue.
I managed to import my NSR wallet after changing the passphrase - B&C exchange may have a bug where it doesn’t properly handle certain special characters in passwords, so once I changed this it imported fine.
However, there was no BKS balance on this wallet, so I’m importing private keys from Coinomi, as I wonder if I had my NSR on there at the time of the issue. Not sure if importing private keys from there will work for this issue, but worth a shot!
If I only had 10k NSR at the time, it’s no big loss, but if I can find a free BKS I will do it, despite how long it takes to import private keys into the NSR wallet 
Thanks for the help, and it’s great learning a bit more about how this all works!